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The plastic six-pack – Artificial muscle can be built from layers of polymer film

By Andy Coglan

THIN slivers of a plastic developed by chemists in Spain and Brazil could
lead to the development of artificial muscles for robots and eventually for
humans or animals. Just 3 micrometres thick, the plastic mimics the behaviour of
fibres in our own muscles, which change shape in response to electrical
signals.

Although artificial muscles have been tried before, they had to be immersed
in a liquid electrolyte. This is the first artificial muscle to work in air.

Toribio Otero and his colleagues in the laboratory of electrochemistry at the
Basque University in San Sebastián, Spain, hope to use the muscles first
in the robots used for delicate tasks in keyhole surgery. “In twenty to fifty
years, we might even be able to make them into artificial muscles for transplant
into humans or animals who need them,” says Otero.

He builds each artificial muscle from two slivers of polypyrrole, a plastic
material that conducts electricity. Sandwiched between these slivers, which act
as electrodes, is a newly developed solid electrolyte, which allows ions to
shuttle from one electrode to the other when an electric current is fed in.

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To make the electrolyte, Marco-Aurelio De Paoli and his colleagues at the
Institute of Chemistry, part of the State University of Campinas, Brazil,
blended lithium perchlorate with a polymer called poly-(epichlorohydrin-co-ethlyene)
(Chemical Communications, 1997, p 2217).

When electric current is fed into the muscle, perchlorate ions migrate
through the electrolyte from one electrode to the other. The direction of the
ions is dictated by the direction of the current. The net result is that the
volume of one polypyrrole electrode swells with the arriving ions, while the
other contracts. These volume changes make the film bend by as much as 180°.
If the direction of the incoming electric current is reversed, the sliver bends
just as far in the opposite direction.

“We have a change in volume controlled by the current,” says Otero. The size
of the current dictates how far and how fast the muscle bends. Otero says this
phenomenon is valuable in the design of artificial muscles for robots. “The
control is perfect,” says Otero.

He says that components of the film match those in real muscle, where nervous
electrical pulses of between 70 and 160 millivolts cause a fibre called myosin
to change shape. Pulses of between 100 and 2000 millivolts are needed to make
the artificial muscle work. The movement of perchlorate ions in the film mimics
the oxidation of glucose, the chemical process that drives muscles in animals.